Windows Vista SP1 enables support for hotpatching, a reboot-reduction servicing technology designed to maximize uptime. It works by allowing Windows components to be updated (or "patched") while they are still in use by a running process. Hotpatch-enabled update packages are installed via the same methods as traditional update packages, and will not trigger a system reboot.  Other changes:

- Improves patch deployment by retrying failed updates in cases where multiple updates are pending and the failure of one update causes other updates to fail as well.
- Enables reliable OS installation by optimizing OS installers so that they are run only when required during patch installation. Fewer installers operating results in fewer points of potential failure during installation, which leads to more robust and reliable installation.
- Improves overall install time for updates by optimizing the query for installed OS updates.
- Improves robustness during the patch installation by being resilient to transient errors such as sharing violations or access violations.
- Improves robustness of transient failures during the disk cleanup of old OS files after install.
- Improves the uninstallation experience for OS updates by improving the uninstallation routines in custom OS installation code.
- Improves reliability of OS updates by making them more resilient to unexpected interruptions, such as power failure